The long-term goal of this project is to understand the role of cytochrome P450 1A1 (CYP1A1) in toxicity caused by environmental pollutants. Cyp1a1 has a broad tissue distribution, is expressed very early in gestation, and is transcriptionally regulated by the Ah receptor (AHR). The environmental contaminants benzo[a]pyrene (BaP) and dioxin are ligands for the AHR. Toxicity caused by these pollutants is known to be AHR-mediated, BaP requiring metabolic activation and dioxin negligibly metabolized. Evidence is accumulating that CYP1A1 also mediates the toxicity of dioxin. It is increasingly clear that, in the most basic of terms, environmental toxicity of chemicals occurs via two routes: [a] covalent binding of reactive intermediates to cellular macromolecules and/or [b] activation of signal transduction pathways that ultimately influence the fate of specific cell types. With our recent success in making the Cyp1a1 (-/-) knockout mouse line, we are now in the unique position to delineate tissue- and cell type-specific CYP1A1- versus AHR-dependent and - independent modes of toxicity elicited by metabolized (BaP) and nonmetabolized (dioxin) AHR ligands. The work of others has unequivocally shown that about 15% to more than 25% (depending on the tissue) of CYP1A1 is located in the inner mitochondrial membrane (mt1A1), the remaining in the endoplasmic reticulum, i.e. microsomes (mc1A1), and that clear differences in the inducibility profile and substrate specificity exist between mt1A1 and mc1A1. Events very early in the apoptosis cascade (cytochrome c release, BCL2 function) also occur in the inner mitochondrial membrane. Thus, CYP1A1-dependent toxicity may be largely ascribed to one subcellular location of the enzyme versus another. Our hypothesis is that Bap and dioxin-induced toxicity of various tissues is primarily he result of CYP1A1 function. For the next funding period we therefore will [1] assess in the Cyp1a1 (-/-) knockout mouse, compared with the Cyp1a1 (+/+) wild-type, differential toxicity induced by BaP versus dioxin in the [a] bone marrow, [b] liver, and [c]developing embryo; and [2] generate exclusively mitochondrial CYP1A1-containing (mt1A1) and exclusively microsomal CYP1A1 - containing (mc1A1) knock-in mouse lines to dissect the mechanism of liver toxicity further. These studies will greatly enhance our understanding of mtCYP1A1 versus mcCYP1A1- mediated toxicity caused by environmental pollutants and perhaps lead to the design of drugs to shield against such toxicity of CYP1A1 substrates and AHR ligands, in the diet and especially for cigarette smokers and occupationally-exposed workers.